During a lethal murine infection with the gram-positive bacterium, Listeria monocytogenes, a factor appears in the serum that is capable of lysing certain tumor cells in vitro. The levels of this serum cytotoxic factor increase with the progression of morbidity. Neutralization of Listeria-induced cytotoxic factor activity with a monospecific antiserum to recombinant murine tumor necrosis factor (TNF) revealed that the cytotoxic factor was antigenically indistinguishable from natural TNF present in endotoxin-induced tumor necrosis serum. In contrast to a lethal infection, no TNF activity was detected in sera of mice throughout a sublethal infection. However, shortly after initiation of a sublethal infection, mice acquire a greatly enhanced capacity to produce serum TNF in response to an intravenous injection of endotoxin. Cultures of unfractionated spleen cells from mice with ongoing Listeria infections produced TNF when incubated in the absence, but not in the presence, of antibiotics. However, such antibiotic-treated cultures could be stimulated with endotoxin to produce substantially higher TNF yields than spleen cells of uninfected mice. It is also shown that intravenous infusion of anti-recombinant murine TNF IgG into sublethally infected mice results in increased Listeria proliferation in spleens and livers, and ultimately in death from an overwhelming infection.